The present invention generally relates to memory devices for use with computers and other processing apparatuses. More particularly, this invention relates to a nonvolatile or permanent memory-based mass storage device using phase change material in combination with optical sensing as a memory storage substrate. Sensing of data is achieved using optoelectronic cells measuring the change of optical transference of the substrate as a consequence of phase change.
Mass storage devices such as advanced technology (ATA) or small computer system interface (SCSI) drives are rapidly adopting nonvolatile memory technology such as flash memory or other emerging solid state memory technology including phase change memory (PCM), resistive random access memory (RRAM), magnetoresistive random access memory (MRAM), ferromagnetic random access memory (FRAM), organic memories, or nanotechnology-based storage media such as carbon nanofiber/nanotube-based substrates. The currently most common technology uses NAND flash memory as inexpensive storage memory.
Despite all its advantages with respect to speed and price, flash memory has the drawback of limited endurance and data retention caused by the physical properties of the floating gate, the charge of which defines the bit contents of each cell. Typical endurance for multilevel cell NAND flash is currently in the order of 10,000 write cycles at 50 nm process technology and approximately 3000 write cycles at 4× nm process technology, and endurance is decreasing with every process node. Data retention is influenced by factors like temperature and number or frequency of accesses, wherein access can either be read or write and further is not confined to the cell holding critical data but can also encompass any cell in the physical proximity of the cell of interest.
Alternative technologies have evolved around phase change materials, that is, materials that upon exposure to heat will change their physical properties in a controlled manner. A well established example are optical media such as CD-ROM and DVD-ROM that use lasers to melt regions in the surface of the media associated with addresses, wherein the resulting changes in the reflectivity of the surface regions create bit values. Optical phase change is primarily used in rotatable media and provides for very inexpensive mass storage and archival media, though with the drawback of very limited re-write capability. Another area in which phase change is used is only currently emerging to market-suitable maturity. This technology employs heat-induced phase change to alter the physical properties of a chalcogenide alloy of germanium, antimony and tellurium (Ge2Sb2Te5), known as GST, from amorphous to crystalline. The change affects the electrical resistance of a small area within the media, which can be measured as the current/voltage (IV) relation to indicate High or Low (1 or 0) as bit value. Writing is performed by Joule heating, that is, driving current across a resistor and the media to a second electrode on the back of the chalcogenide media. Depending on the temperature and duration of the heat pulse, the chalcogenide substrate changes from the amorphous to the crystalline phase. In contrast to NAND flash, where writing data can only change bit values from 1 to 0, therefore requiring an erase cycle to write “1” values to every cell before re-writing it, PCM can be written in both directions by changing the duration and amplitude of the heat pulse. A heat pulse can also be a light pulse in this context. The advantage is that there is no need for any intermittent erase if data have to be re-written.
Optical signaling has some advantages over electrical signaling and, in fact, optical interconnects are often the media of choice when it comes to high speed interconnects. Likewise, as mentioned above, data storage based on the phase change of optical properties preceded data storage based on the phase change of resistance properties. Because of the versatility of optical signaling, having for example different properties depending on wave length, lower thermal load, and lack of capacitive coupling among other factors, it should be advantageous to have a phase change memory using optical transference differentials as a function of substrate phase change for data storage.